Studies on (a) mouse strain variation in susceptibility to Ascaris suum in terms of lung larvae numbers, (b) acquired resistance to A. suum, (c) T cell activities in the development of resistance to second infection with A. suum, (d) IgM anti phosphorylcholine responses induced by A. suum larvae in mice, (e) inhibition of antibody responses to antigens when complexed with phosphorylcholine, (f) eosinophilia induced by A. suum infections and isolation of circulating eosinophils as a prelude to analysis of therir cell surface markers and immunological activities, (g) induction of mouse reagins by the allergens in the body fluid of adult A. suum worms and development of plague forming cell assays for reaginic antibody-secreting cells, (h) cell mediated immunity to isolated allergens, (i) T cell activities in the induction and maintenance of reagin production and eosinophilia and (j) the role of mucous membrane location of antigen in induction and maintenance of reagin production and eosinophilia.